The peptide hormones, oxytocin and arginine vasopressin have been synthesized specifically enriched (90%) with 13C in various carbon atoms. The interactions of these hormones with purified bovine neurophysins have then been studied using 13C NMR spectroscopy. Among the positions which have been enriched are the alpha-carbons of cys-1, tyr-2, and leu-3 (in 3-leucine oxytocin), the carbonyl carbons of cys-1 and cys-6, and the 3',5' (meta) positions of tyr-2. Upon hormone binding to neurophysins, large chemical shifts are seen for the resonances due to these carbons. Studies at various temperatures and pH values have been performed, as well as studies with various D-stereoisomers of oxytocin.